Energy spectrometry, e.g. nuclear and x-ray spectrometry evaluates a nuclear or x-ray flux by the collection of electric charges released in a detector by ionization of the detector material by the quanta of nuclear and x-ray radiation entering this material. The liberated electric charge is proportional to the energy of the radiation quanta absorbed in the detector.
The liberated charge is transformed by a charge-responsive preamplifier into a charge-proportional or energy-proportional voltage step which is subjected to filtering in a low-pass filter to reduce the electronic noise contributed to the signal by the preamplifier. The filter can be of the analog of digital type.
Spectrometry of this kind is the subject of my U.S. Pat. No. 4,476,384 and reference may be had also to the publications cited in the file of that patent (see also M. Bertolaccini et al: Nuclear Instruments and Methods 61 (1968) pages 84 ff; GP Westphal: Nuclear Instruments and Methods 138 (1976) pages 467 ff and 163 (1979) pages 189 ff; N Taccetti: Nuclear Instruments and Methods 225 (1984) pages 118 ff).
In United Kingdom patent publication No. 2,116,388A, there is disclosed a variable circuit characteristic which can be incorporated possibly as a filter characteristic and in which the switch is controlled with a signal having a variable keying ratio and which either passes or blocks the selected components of the signal. This allows a stepped or continuous control of a filtering process. The switch controlling signal in this case is generally not related to the signal to be filtered but must have a higher frequency than the latter.
U.K. patent publication No. 2,081,543A describes a commutated resistance circuit in which the control is effected generally in the same manner as in U.K. No. 2,116,388A, i.e. the control is by means of a control signal with a continuously variable keying ratio. The control signal generates a continuously variable intermediate value between two resistances. The higher frequency-control frequency of the switch is suppressed by means of a condenser.
The German patent publication DE No. 3,132,479A1 discloses a N-path filter in which the switch is controlled by three to N temporally overlapping phases. The German patent document DE No. 3,301,792A1 shows a switched condenser chain with reduced capacity in which, from a filtering input signal, a filtering output signal is generated by sampling at a predetermined sampling rate. This is effected by a condenser, a reversing or switchover circuit and an integrating circuit.
German patent document DE No. 3 301 656 discloses an arrangement for simulating electrical components.
It is common to all three of these German patent documents that the filter characteristic, especially the cutoff frequency, is determined by the respective circuit arrangement or can be varied by changing the control frequency.
To further improve the signal-to-noise ratio of the measurement in spectrometry of the class described, it is essential to reduce the electronic noise of the preamplifier and it is for this reason that a bandwidth limitation must be introduced by a suitable low-pass filter. At the same time a shortening of the duration of the voltage steps by electronic differentiation is required to avoid overlap of the steps and should be able to process a sufficient number of successive events in unit time.
By differentiation and multiple low-pass filtering of the voltage steps, signals of a quasi constant semi-Gaussian shape and of a variable amplitude are produced, the amplitude being proportional to charge and energy. This is the most representative example of nuclear pulse shaping.
A drawback of the conventional pulse shaping approach is the fixed duration of the filtering process of a single event which is determined by the filters step response. Signals with a closer spacing than the filters step response stack up and their amplitude information is lost. This phenomenon, also described below, is termed pulse-pileup.
The pulse-pileup phenomenon is the greatest obstacle in the processing of high counting rates and the most common approach in the past has been to use a filter with a short space response and consequently less efficient noise reduction or to limit the counting rate where high noise reduction is required.
Significantly better results have been achieved recently by a switched integration technique. In this case, the desired low-pass effect is obtained by the combination of a semi-Gaussian filter of comparatively short duration and the switched integrator to which it is connected.
The output of this system is comparable to the rising flank of a conventional semi-Gaussian filter. It however turns immediately downwardly towards zero by contrast with the characteristic of the semi-Gaussian filter upon discharge of the integrator and thus is free to respond promptly to subsequent signals.
The disadvantage of all heretofore known filter techniques is that the instantaneous value of the filter output builds up from a starting value of zero continuously to the energy-proportional final value which requires the aforementioned certain fixed duration for the filters response. It is this which causes the pulse-pileup upon the detection of signals with shorter spacing than the recovery time of the filter.
Thus, where pulse-pileup occurred with earlier filters, the additional pulses, although detected, were subjected to pulse-pileup rejection, i.e. were nonacknowledged events.
This has been the problem especially for signal or event trains which an exponentially shorter interval between events with increasing counting rates as is the case for measurements of radiation from radioactive wastes, and in other nuclear or x-ray spectrometry applications in which with increasing counting rates there is an increasing count loss.